The present invention relates to a process for the production of partially fluorinated fluoropolymers, in accordance with which fluoromonomers are free-radically polymerized in the presence of polyfluoropropanes or polyfluorobutanes of the formulae CF3xe2x80x94CH2xe2x80x94CF2H, CF3xe2x80x94CHFxe2x80x94CF2H, CF3xe2x80x94CH2xe2x80x94CF3, CHF2xe2x80x94CF2xe2x80x94CH2F, CF3xe2x80x94CH2xe2x80x94CF2xe2x80x94CH3 and CF3xe2x80x94CHFxe2x80x94CF2xe2x80x94CH3 and optionally a chain-transfer agent.
Fluoropolymers are produced on an industrial scale substantially using known aqueous emulsion or suspension polymerization processes, as described, for example, in Modern Fluoropolymers, John Wiley and Sons Ltd., Chichester, 1997, pp. 77 and 609. Water-soluble auxiliaries, such as fluorinated emulsifiers, dispersants, initiators etc. are required in these processes which, once the resultant polymer dispersion has been worked up, may be found in part in the product and in part in the waste water, where they cause problems with regard to product properties or the environmental compatibility of the process.
Non-aqueous processes have hitherto been based upon chlorofluorocarbons, such as 1,2-dichlorotetrafluoroethane or 1,1,2-trichlorofluoroethane. However, these compounds have an elevated ozone degrading potential, for which reason industrial use thereof is already prohibited in many industrial nations.
U.S. Pat. No. 4,243,770 mentions the possibility of using other inert compounds containing fluorine, such as perfluoro(1,2-dimethylcyclobutane), perfluorocyclohexane, perfluoro(tributylamine) and compounds of the type H(CF2)nH and CF3O(C2F4O)nCF2CF3, apart from the stated chlorofluorocarbons, as solvents for the polymerization of fluoromonomers.
U.S. Pat. No. 5,182,342 describes the use of fluorinated hydrocarbons which satisfy certain criteria with regard to the F/H ratio and the position of the hydrogen atoms and may optionally contain ether oxygen atoms for this purpose, such as for example 1,1,2,2-tetrafluorocyclobutane, 1-trifluoromethyl-1,2,2-trifluorocyclobutane, CF3(CF2)nCFH(CF2)mCF3, CF3(CF2)nCFHCFH(CF2)mCF3, CF3(CF2)nCH2(CF2)mCF3, CF3(CF2)nCFHCH2(CF2)mCF3, C4F9CH2CH2C4F9. However, compounds which, with the exception of the permitted structural unit xe2x80x94CF2OCH3, contain a hydrogen atom on the terminal (primary) C atom are explicitly excluded.
Fluorinated hydrocarbons having terminal hydrogen atoms, for example CF3(CF2)nH, CF3(CF2)nCH2CH3, as well as those of the type CF3CF(CF3)CFHCFHCF3, are described in U.S. Pat. No. 5,494,984 in addition to those mentioned above as polymerization media for fluoromonomers.
It has not hitherto been possible to use fluorinated hydrocarbons having fewer than 4 C atoms in known processes for the production of fluoropolymers. The compounds hitherto used have also been very highly fluorinated compounds, in which partially fluorinated fluoropolymers, which also comprise xe2x80x94CH2 sequences in addition to fluoro-substituted carbon atoms, are very sparingly soluble. Moreover, on grounds of price and availability, none of these compounds is suitable as a polymerization medium. The production thereof generally proceeds by oligomerization/telomerisation of fluoromonomers (tetrafluoroethylene, hexafluoropropene, hexafluoropropene oxide) and subsequent hydrogenation or hydrogen fluoride addition.
However, increasing the H/F ratio as a pre-requisite for improving polymer solubility generally results in an increase in the transfer potential, which is undesirable for a polymerization solvent, as this latter property is usually also associated with a terminating chain transfer.
The object of the present invention was accordingly to provide a simple production process for partially fluorinated fluoropolymers which operates without using ozone-damaging compounds.
It has now been found that partially fluorinated fluoropolymers may be produced in a very simple manner by using certain polyfluoropropanes or polyfluorobutanes.
The present invention provides a process for the production of partially fluorinated fluoropolymers, in accordance with which fluoromonomers are free-radically polymerization in the presence of fluorinated hydrocarbons of the formulae CF3xe2x80x94CH2xe2x80x94CF2H, CF3xe2x80x94CHFxe2x80x94CF2H, CF3xe2x80x94CH2xe2x80x94CF3, CHF2xe2x80x94CF2xe2x80x94CH2F, CF3CH2xe2x80x94CF2xe2x80x94CH3 and/or CF3xe2x80x94CHFxe2x80x94CF2xe2x80x94CH3, preferably 1,1,1,3,3-pentafluoropropane, and optionally a chain-transfer agent.
Partially fluorinated fluoropolymers for the purposes of the invention have a main chain formed of carbon atoms, which chain also comprises xe2x80x94CH2 sequences as well as fluoro-substituted sequences. These may be either homo- or copolymers of vinylidene fluoride with other fluorinated and also non-fluorinated monomers or copolymers of fluorinated monomers, containing no hydrogen, with non-fluorinated monomers.
Fluoromonomers which may be used for the purposes of the invention are fluorinated, optionally substituted ethylenes, which, apart from fluorine, may contain hydrogen and/or chlorine, such as for example vinylidene fluoride, tetrafluoroethylene and chlorotrifluoroethylene, fluorinated 1-alkenes having 2-8 carbon atoms, such as for example hexafluoropropene, 3,3,3-trifluoropropene, chloropentafluoropropene, hexafluoroisobutene and/or perfluorinated vinyl ethers of the formula CF2xe2x95x90CFxe2x80x94Oxe2x80x94X, where X=C1-C3 perfluoroalkyl or xe2x80x94(CF2xe2x80x94CFYxe2x80x94O)nxe2x80x94RF, wherein n=1-4, Y=F or CF3 and RF=C1-C3 perfluoroalkyl.
Olefins containing no fluorine, such as ethylene, propene, isobutene, alkyl vinyl ethers or vinyl esters, such as for example vinyl acetate, which are copolymerizable with fluoromonomers may furthermore additionally be used as comonomers.
Examples of such copolymer combinations are, for example, vinylidene fluoridelhexafluoropropene, optionally together with tetrafluoroethylene and/or perfluoro(methyl vinyl ether), tetrafluoroethylene/ethylene, tetrafluoroethylene/vinylidene fluoride, tetrafluoroethylene/propene, tetrafluoroethylene/propene/vinylidene fluoride and tetrafluoroethylene/vinyl acetate.
It is additionally possible also to use copolymerizable monomers containing iodine or bromine, such as for example bromotrifluoroethylene, 4bromo-3,3,4,4-tetrafluoro-1-butene, as described in U.S. Pat. No. 4,035,565, or 1-bromo-2,2-difluoroethylene for the production of peroxide-vulcanisable fluororubbers.
In a preferred embodiment of the invention, water is additionally present in a quantity of 10 to 900 parts by weight relative to 100 parts by weight of the polyfluoropropenes or polyfluorobutanes used according to the invention.
The free-radical polymerization is preferably initiated by means of initiators.
Organic or fluoro-organic dialkyl peroxides, diacyl peroxides, dialkyl peroxy-dicarbonates, alkyl peresters and/or perketals, such as for example tert.-butyl peroxypivalate, tert.-butyl peroxy-2-thylhexanoate, dicyclohexyl peroxydicarbonate, bis(trifluoroacetyl peroxide) or the peroxide of hexafluoropropene oxide dimers {(CF3CF2CF2O CF(CF3)COO}2 are preferably used as the initiator. The initiator which is used and the quantity in which it is used is determined by the particular reaction temperature, at which the half-life of the peroxide to be selected should be between 30 and 500 min. Quantities of between 0.05 and 1.0 part by weight of peroxide per 100 parts by weight of monomers to be reacted are preferred.
When the polymerization is performed as suspension or emulsion polymerisation in the presence of water, which is additionally added to the fluorinated hydrocarbons according to the invention, it is also possible to use water-soluble initiators or redox systems, in which one or both of the components (reducing and oxidizing agent) are water-soluble, such as for example potassium peroxydisulfate, potassium permanganate/oxalic acid or peroxydisulfate/perfluoroalkyl sulfinate.
The molecular weights and thus the viscosities of the desired products may be adjusted by means of the quantity of initiator or by addition of one or more chain-transfer agents.
Preferably used chain-transfer agents are compounds of the formula (III)
R3BraIbxe2x80x83xe2x80x83(III)
where a or b=0 to 2 and a+b=1 or2, wherein R3 may be an aliphatic hydrocarbon, fluorinated hydrocarbon, chloro-fluorocarbon or fluorocarbon residue having 1-8 carbon atoms. These may comprise, for example, 1,2-dibromo-1-chlorotrifluoroethane and/or 1-bromo-2-iodotetrafluoro-ethane. More preferred diiodo-organic compounds are those in which both iodine atoms are attached to the same or to different carbon atoms. Most preferred hydrocarbon or fluorocarbon compounds are those having 1 or 4 carbon atoms, wherein the iodine is located on the terminal carbon atoms. Diiodomethane and/or 1,4-diiodoperfluorobutane are likewise very preferred. The quantity of diiodo-organic compound is preferably 0.1-3.0 parts by weight of iodine per 100 parts by weight of polymerized fluoromonomer.
The polyfluoropropanes or polyfluorobutanes used in the process according to the invention preferably have an H/F ratio of 1/3 to 3/3.
Thanks to the particular distribution of the hydrogen atoms, it is ensured that no transfers from the solvent occur which impair the reaction or product properties. If, for example, 1,1,1,4,4,4-hexafluorobutane is used instead of the 1,1,1,3,3-penta-fluorobutane used according to the invention, considerable retardation of polymerization is observed.
The polyfluoropropanes or polyfluorobutanes used according to the invention are produced using known processes (c.f. for example Zh. Org. Khim. 1980, 1401-1408 and 1982, 946 and 1168; Zh. Org. Khim. 1988, 1558; J. Chem. Soc. Perk, 1, 1980, 2258; J. Chem. Soc. Perk Trans., 2, 1983, 1713; J. Chem. Soc. C 1969, 1739; Chem. Soc. 1949, 2860; Zh. Anal. Khim. 1981 36(6), 1125; J. Fluorine Chem. 1979, 325; Rosz. Chem. 1979 (48), 1697, J. Amer. Chem. Soc. 67, 1195 (1945), 72, 3577 (1950) and 76, 2343 (1954)). Since the boiling points of these fluorinated hydrocarbons are between approx. xe2x88x921 and 40xc2x0 C., they may readily be removed from the product on completion of polymerization. Any possible secondary products, which may arise from the initiator and/or the chain-transfer agents, have higher boiling points, such that the solvent may be recovered again in very pure form.
The reaction temperatures for the free-radical polymerization are preferably between 30 and 130xc2x0 C. Lower temperatures result in a dramatic extension of running time and in a sharp increase in the viscosity of the polymer solution, such that problems may occur with regard to mass transfer, heat dissipation and product discharge. The space-time yield cannot be raised substantially firther with still higher temperatures, while product properties are degraded. A preferred temperature range for polymerization is 60-100xc2x0 C.
The pressure during polymerization is dependent upon the above-stated conditions and upon the composition of the monomer mixture and is preferably between 10 and 100 bar. The process according to the invention is more preferably performed at pressures of between 15 and 50 bar.
The free-radical polymerization may be performed by batch, continuous or batch/feed processes in stirred tank reactors, wherein the batch/feed process is preferred.
Once polymerization is complete, the reaction mixture may readily be discharged or expressed from the tank via a bottom discharge or riser pipe. Residual monomers and the solvent may then readily be separated from the polymer by releasing the pressure.
The polymer solution may, however, also be used without further working up as a coating material, which use is also provided by the present invention. Substrates which may be considered for coating are, for example, metals, plastics, textiles, leather, paper and nonwovens.
Due to their low boiling points, the solutions according to the invention may also readily be sprayed onto substrate surfaces by intrinsic pressure. If curing is required after application, the coatings according to the invention may be vulcanized using known methods, for example by free-radical methods by means of co-vulcanizing agents and light or peroxides, with polyamines or polyols and with the assistance of diisocyanates, if the polymer contains hydroxyl groups.
Fluororubbers produced using process according to the invention may be compounded and vulcanized using conventional methods, c.f. Modern Fluoropolymers, John Wiley and Sons Ltd., Chichester, 1997, pp. 78, 115, 601. Thanks to their low viscosity, the fluororubbers may be firther processed to yield elastic moldings by using advantageous injection moulding techniques.
Compounds suitable for vulcanization are bisnucleophiles, such as bisamines, for example hexamethylenediamine, or bisphenols, for example 2,2-bis(4-hydroxy-phenyl)hexafluoropropane (xe2x80x9cBisphenol AFxe2x80x9d) in combination with vulcanisation accelerators, such as quaternary phosphonium, ammonium or sulfonium salts and acid acceptors, such as magnesium oxide and calcium hydroxide, c.f. A. L. Logothetis in Polym. Sci. 14 (1989) 251-296 and the literature cited therein. Alternatively, fluororubbers which have been produced by the process according to the invention and contain bromine and/or iodine covalently bonded to the carbon main or side chain, may be vulcanised by organic peroxides such as 2,5-dimethyl-2,5-bis(tert.-butyl)hexane in combination with co-vulcanising agents such as triallyl isocyanurate (c.f. for example EP-A 398 241).
The present invention also provides the use of 1,1,1,3,3-pentafluoropropane as process solvent for the polymerization of fluoromonomers.